The invention relates to a method of and an arrangement for determining the individually controllable amounts of light in various colors to which photographic copying material is to be exposed in a color copying operation from a length of photographic original material including a series of discrete image-carrying sections, especially from a film strip including a series of color negatives. More particularly, the present invention relates to a method and an apparatus of the above type wherein the density of the original material in three basic colors is individually photoelectrically measured at a plurality of regions of the original material, and wherein the results of measurements are used for controlling the amount of copying light in the color copying operation.
A widely used method for determining the amounts of copying light is based on the recognition of the fact that, in most instances, satisfactory color copies are obtained when the amounts of copying light in the three colors are so controlled that a neutral gray results in the copy independently of the color composition of the original being copied. This assumption is substantially valid as long as no large areas of the same color, so-called color dominants, are present in the original from which the copy is to be made. On the other hand, when the original has such a dominant, this dominant is suppressed by very non-uniform measuring of the copying light amounts in the individual colors to such an extent that again neutral gray is obtained for the copy. Therefore, in the event of existence of such a color dominant, the neutral gray compensation which is well suited for the elimination of color casts, results in improper exposure of the copying material to the light in the various colors during the copying operation and, hence, the copies from originals including such color dominants are unsatisfactory.
In a color copying method described in the German published application Ser. No. 19 14 360, the neutral gray compensation is not limited to a single negative; rather, it is applied to a large number of originals which are treated in the same way as, for instance, within the same film strip. In this manner, there is obtained a compensation for a color cast or tinge which occurs in the particular film strip or in a large number of consecutive negatives of the same film strip. The existence of the aforementioned tinge or cast may be attributable, for instance, to the peculiarities of the film emulsion, to the changes which may take place during the storage of the film strip prior to or subsequent to exposure, or to errors occurring during the development of the original film strip. Then, the color dominants which are present in the individual image-carrying sections, for instance negatives, and whch are attributable to the theme of the respective image, are reproduced in a satisfactory manner, inasmuch as such color dominants usually appear, in the same color, only in some of the color-carrying sections of the original material and, therefore, the influence of such color dominants is hardly felt in the summation of the individual color values. However, experience has shown that a control of the color exposures only based on the averaging of the results of measurements applicable to the entire film strip, which is completely suitable in many instances, does not achieve satisfactory results for all possible original materials, especially when such original materials include image-carrying sections which have drastically different average densities. This may be attributable to the fact that, in most instances, the photosensitive materials have color errors which are dependent on the density.